1. Field of the Invention
This invention relates to a radiation image read-out method wherein final read-out is carried out for obtaining image signals for use in reproduction of a visible image from a recording medium such as a stimulable phosphor sheet carrying a radiation image recorded thereon, image processing conditions for use in processing of the final read-out image signals obtained by the final read-out are adjusted on the basis of the final read-out image signals, and the final read-out image signals are processed by use of the image processing conditions, and an apparatus for carrying out the method. This invention particularly relates to a radiation image read-out method wherein the image processing conditions are accurately adjusted in the case where the radiation image recording is carried out by subdivision image recording, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected to obtain final read-out image signals for use in reproduction of a visible image, image processing is carried out on the final read-out image signals, and the radiation image of the object is reproduced as a visible image by use of the processed final read-out image signals on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In this specification, detection of the image signals for use in reproduction of the visible image is referred to as the final read-out, and the image signals obtained by the final read-out are referred to as the final read-out image signals. The terms "final read-out" and "final read-out image signals" are used in the same senses also in the case where the recording medium is of the type different from the stimulable phosphor sheet.
In general, the image processing in the aforesaid radiation image recording and reproducing system is carried out by use of image processing conditions adjusted for each image in accordance with the image recording portion of the object and/or the image recording method to obtain a visible image suitable for viewing, particularly for diagnostic purposes. However, it may be considered that the image processing conditions be adjusted on the basis of, for example, the final read-out image signals (a histogram of the final read-out image signals or the like), instead of the image recording portion of the object and/or the image recording method, or on the basis of both the final read-out image signals and the image recording portion of the object and/or the image recording method.
In the case where the image processing is carried out by use of the image processing conditions adjusted on the basis of the final read-out image signals which directly represent the nature of each radiation image, it becomes possible to obtain a visible image always having an improved image quality, particularly a high diagnostic efficiency and accuracy, i.e. a visible image wherein the necessary object image information is always expressed within the density range suitable for viewing, particularly for diagnostic purposes, regardless of any fluctuation in the level of the radiation energy stored on the stimulable phosphor sheet, which is caused by changes in the object, the image recording portion thereof, radiation dose, or the like.
In the case where a radiation image is recorded on the stimulable phosphor sheet, subdivision image recording is often carried out. In subdivision image recording, the recording region on the stimulable phosphor sheet is divided into a plurality of subdivisions, and the respective subdivisions are exposed to radiation for image recording. Subdivision image recording is economical since, for example, when an image of a small object portion is recorded on a large stimulable phosphor sheet, images of a plurality of object portions may be recorded on a single stimulable phosphor sheet. Also, the radiation image recording and read-out processing speed becomes high.
However, in the case where subdivision image recording as mentioned above is carried out, the final read-out image signals may not accurately represent the natures of the actually recorded images. Specifically, as shown in, for example, FIG. 2, in the case where the number of the subdivisions at which the image recording is actually carried out (three subdivisions in this example) is smaller than the total number of the subdivisions to which the recording region on the stimulable phosphor sheet is divided (four subdivisions in this example), the final read-out image signals also include read-out signals detected from the subdivision at which no image is recorded. Therefore, the final read-out image signals, for example a histogram thereof, used for ascertaining the natures of the images (image input information) do not correctly represent the radiation images actually stored on the stimulable phosphor sheet.
In such a case, even though the image processing conditions are adjusted based on the final read-out image signals, it is not always possible to adjust the image processing conditions to appropriate values.
Besides the case of the image recording wherein the stimulable phosphor sheet is utilized, the aforesaid problems with regard to the adjustment of the image processing conditions arise generally in the case where radiation images are recorded on a recording medium by subdivision image recording.
By the term "image processing conditions" as used herein are meant various conditions affecting the relationship between the input to the image processing means and the output thereof, for example, gradation processing conditions and frequency response processing conditions.